“A Great Solar Cell Has To Be a Great LED”; So What’s Wrong With Subsidized Solar Panels?

MIT Energy Initiative IHS Seminar, Cambridge Mass. 5PM, Wednesday Feb. 8, 2017

Eli Yablonovitch, Vidya Ganapati, Patrick Xiao Electrical Engineering and Computer Sciences Dept., Univ. of California, & Lawrence Berkeley Laboratory 30

29 Alta Devices record, 28.8%

28 1-sun, single junction, 27 solar cell efficiency record:

26 Efficiency (%) Efficiency

25

24 1990 1995 2000 2005 2010 Year Open Circuit voltage V (Volts)

0.95 1.00 1.05 OC 1.10 1.15 1990 Alta Devices record cell, 1.122 volts record efficiencycells: Open Circuitvoltagefor 1995 2000 Year 2005 2010 GaAs solar cells are the preferred technology, where cost is no objection: Space The Epitaxial Liftoff Process:

Courtesy of Alta Devices, Inc. Li Hejun h h h

25.1% e- efficiency + 1990-2007 h

h h h hg h g

28.8% e- efficiency

2011-2012 h+ excellent reflector >>95% Shockley told us to generate the maximum possible external luminescence:

qVoc = qVoc-ideal – kT|ln{ext}|

The external luminescence yield  Only external ext is what matters! Luminescence can balance the incoming radiation. 1 sun results from Alta Devices, Inc.

Expected to reach 29.8% single junction For solar cells at 25%, good electron-hole transport is already a given.

Further improvements of efficiency above 25% are all about the photon management!

A great solar cell needs be a great LED!

Counter-intuitively: Solar cells perform best when there is maximum external fluorescence yield ext.

Miller et al, IEEE J. Photovoltaics, vol. 2, pp. 303-311 (2012) Dual Junction Series-Connected Tandem Solar Cell

h h

n-Al0.5In0.5P Eg~2.35eV Ga0.5In0.5P Ga0.5In0.5P VOC=1.5V n-Ga0.5In0.5P Eg~1.8eV VOC=1.5V Solar Cell Solar Cell p-Ga0.5In0.5P Eg~1.8eV + Tunnel p -Al0.5In0.5P Eg~2.35eV Tunnel + Contact n -Al0.5In0.5P Eg~2.35eV Contact n-Al0.5In0.5P

GaAs n-GaAs Eg=1.4eV GaAs V =1.1V V =1.1V OC p-GaAs Eg=1.4eV OC Solar Cell Solar Cell p-Al0.2Ga0.8As

All Lattice-Matched ~34% efficiency should be possible. Dual-junction 1 sun results from Alta Devices, Inc.

ALTA has demonstrated >31.5% efficiency in the same system.

Expected to reach 34% dual junction, eventually. 38.8% Efficient--all time champion solar cell Quadruple-junction 1-sun cell captures diffuse & direct light

designed for Luminesce- total Extraction & internal Photon- reflection Recycling

IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 6, p.358 (JANUARY 2016) Myles A. Steiner, Sarah R. Kurtz, et al, NREL USA Counter-Intuitively, to approach the Shockley-Queisser Limit, you need to have good external fluorescence yield ext !!

Both Internal Fluorescence Yield  >> 90% int needed for Rear reflectivity >> 90% good ext What is happening in the solar economy? c-Si  ~ 15%-23% in production 90% market share 75GW/year annual world-wide production capacity World-wide demand ~60GW/year Oversupply! The current world price has settled at $0.40-0.50/Watt Learning Curve 21.5% per 2X production (1976-2014) Learning Curve 21.5% per 2X production (1976-2014)

silicon shortage

over-capacity Learning Curve 21.5% per 2X production (1976-2014)

silicon shortage

Shipments Average Price 2015 50GWp $0.58/Wp over-capacity 2016 60GWp $0.48/Wp

Jan. 2017 Spot Price $0.43/Wp

Cumulative world-wide >300GWp installed Friday Saturday

After spending ~1011Euros, Germany has installed 40GW of panels, but receives only 7% of its electricity from solar What is happening in the solar economy? c-Si  ~ 15%-23% in production 90% market share 75GW/year annual world-wide production capacity World-wide demand ~60GW/year Oversupply! The current world price is diving to $0.40-0.50/Watt but cost is > $0.60-0.70/Watt

Two TeraWatts can be built out in 30 years, requiring no additional capacity. To justify additional production capacity New and different application markets are needed. For Photovoltaics to make a further impact, new applications and markets are needed; bigger than the 10% impact on the electric utility industry. 1. Pumped water for Reverse Osmosis desalination.

2. Solar Fuels:

3. Thermo-PhotoVoltaics 4. Electro-Luminescent Refrigeration & Heat Engines Since we are for the first time making really efficient photovoltaic cells (also LED's), some new ideas become very timely

What is Thermo-PhotoVoltaics? What is Thermo-PhotoVoltaics?

4 Blackbody Power Spectrum x 10 Differential Blackbody Power 909

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0 0 0 0.2 0.4 0.6 0.8 1 1.2 0 0.2 0.4 Photon Energy0.6 [eV] 0.8 1 1.2 Photon Energy [eV] Thermo-PhotoVoltaic Hybrid Car:

1997 Only ~20% efficiency

Proceedings Future Transportation Technology Conference, Christ, S. and Seal, M., "Viking 29 - A Thermophotovoltaic Hybrid Vehicle Designed and Built at Western Washington University," SAE Technical Paper 972650, 1997, doi:10.4271/972650. Superb Rear Reflector; Recycle the Infrared Photons:

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0 0 0 0.2 0.4 0.6 0.8 1 1.2 0 0.2 0.4 Photon Energy0.6 [eV] 0.8 1 1.2 Photon Energy [eV] Default Solution: engineer the emissivity spectrum to preferentially produce

big photons, h>Eg ~50 years of research

Recent developments in high-temperature photonic crystals for energy conversion Veronika Rinnerbauer *a, Sidy Ndao bc, Yi Xiang Yeng ab, Walker R. Chan ab, Jay J. Senkevich b, John D. Joannopoulos ab, Marin Soljačić ab and Ivan Celanovic b aResearch Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA

Energy Environ. Sci., (2012), 5, 8815-8823 Thin-Film GaAs Solar Cell: Reflectivity Above and Below BandGap

100 Sub-bandgap reflectivity > 92%

Sample AD19631

above-bandgap anti-reflection coating thermophotovoltaic chamber

reflected thermal water cooling radiation on all exterior surfaces hot source

photovoltaic cells lining inner faces of radiation chamber

280 suns bouncing around internally! Small area photovoltaic cell is adequate. Convert Heat to Electricity with >50% Efficiency 70

60

50

40

30 Burn Fuel 20 Single Junction Cells but 2X 1500°C = Radiation Temperature efficiency 10 20°C = Cell Temperature and half the  not SQ limit; non-ideality ext=0.3 emissions 0 Conversion of Heat to Electricity Efficiency (%) Electricity to of Heat Efficiency Conversion 90% 91% 92% 93% 94% 95% 96% 97% 98% 99% Photovoltaic Cell Infrared Reflectivity in % Thermo-PhotoVoltaic Hybrid Car:

50kWatt—70cm  70cm 1200C is equivalent to 100suns to 500suns 1056 1056 Watts hot water 1000 Watt electricity home For use: watercoolingon all exteriorsurfaces

dollar bill for size scale 20cm

thermal radiation chamber fuel heated reaction manifold air exhaust photovoltaic cells lining inner faces of radiation chamber radiation thermal Quad-Copters for civilian & military use: : Duration depends on energy density Lithium battery lasts 20 minutes.

Liquid fuel has 50 times higher energy density, would last 16 hours. For Deep Space use, heat Source can be nuclear, SiC pellets at 1500C. But there is competition from

Fuel Cell vehicles; 2H2+O22H2O

requires H2 storage; (but new H2 storage technologies are being invented) Since we are for the first time making really efficient photovoltaic cells (also LED's), some new ideas become very timely

Electro-luminescent refrigeration (thermophotonic cooling)

My student Patrick Xiao Traditional Thermoelectric cooler/generator

electric current carries heat phonon energy required to surmount barrier phonon energy Metal n-Bi2Te3 returned to the heat bath

this side gets cold Metal this side gets hot

electric current drags entropy  from left to right

Also works to generate electricity The hot side sends out more electrons than the cold side Thermoelectric cooler

TC TH Phonons Phonons Thermal Joule conduction, κ Joule heating heating ρ ρ resistivity J resistivity

Peltier effect,

αEF/T

Semiconductor

+ The efficiency is maximized by optimizing – 훼2 푍 = 휌휅 State of the art: ZT = 1, (T = 300K)  ~10% of Carnot limit h h h *ask any astrophysicist

h h

solar cell

Free Energy = h– TS

qVoc = Eg– TS

For GaAs Eg is 1.4eV But the record Voc=1.12Volts

Most of the entropy is due to loss of directionality information. Now reverse everything h h LED light carries h away entropy S h h

light emitting diode input 1.12 Volts

get out Eg= h =1.4eV Where does the extra 0.28eV come from? obviously heat from the lattice.

Most of the LED light entropy is due to loss of directionality information. Electro-luminescence pumps out 0.3eV of heat/photon

Supplied LED work, W Temperature T Phonons

Entropy carried J Entropy flux per photon + V – Photon flux, Φ

Heat pumped out of LED = > 0

Heat pumped per Heat generated per failed emitted photon, 0.3 eV luminescence, 1.1 eV

This means: we need at least ηext > 80%! Or much higher for good performance 42 Shockley's perpetual motion machine, circa ~1955

LED PV cell Phonons Phonons

+ + ϕLED V JLED JPV VPV LED Photon flux – –

"Thermal Energy Taken from Surroundings in the…Radiation from a p-n Junction Jan Tauc, Czechoslovak J. Phys. 7, 275 (1957) Electro-Luminescent Heat Engine: TC  TH Convert heat to electricity, or electricity to refrigeration

LED, TC PV, TH Phonons Phonons

+ + J ϕLED V VLED LED JPV PV ϕPV – – Photon flux

ℎ휈 − 푉퐿퐸퐷 ∙ 퐽퐿퐸퐷 푄퐶 푞 퐽퐿퐸퐷 = 퐽푃푉 = 푞 휙퐿퐸퐷 − 휙푃푉 퐶푂푃 = = 푊 푉퐿퐸퐷 ⋅ 퐽퐿퐸퐷 − 푉푃푉 ⋅ 퐽푃푉 The LED and the PhotoVoltaic Cell are really the same device.

LED PV 280K 330K

100 nm 100 nm h h h

25.1% e- efficiency + 1990-2007 h

h h h hg h g

28.8% e- efficiency

2011-2012 h+ excellent reflector >>95% This all works because GaAs is the most efficient fluorescent material that can be electrically pumped.

99.7% internal luminescent efficiency has been documented.

For Yb:YLF, Ytterbium in Yttrium Lithium Fluoride, 99.9% luminescent efficiency is known. GaAs LED light extraction design controls performance Low index, n Ag Net reflectivity of the rear mirror 100% 100% angle-averaged, polarization-averaged 99.9% 99.8% 99.8%

99.6% MgF2 99.6%

SiO2 99.4% 99.4%

• Evaluated at hν = 1.42 eV Rear reflectivity (%) reflectivity Rear • Angle-averaged, polarization-averaged 99.2% effective rear reflectivity: 99.2%

• SiO2: 99.91% • MgF2: 99.93% 99% 99% 0o 15o 30o 45o 60o 75o 90o Angle in high-index medium external luminescence efficiency of LED 100%

99%

98% MgF2, ηext = 97.40% 97% SiO , η = 96.78% 96% 2 ext

95% GaAs 94% luminescence efficiency luminescence Low index, n 93% Ag 92% External • Internal luminescence efficiency ~ 99.7% 91% • LED thickness = 100 nm • Rear surface texturing 90% 1 1.1 1.2 1.3 1.4 LED bias voltage (V) We can actually do better than a perfect rear mirror: ideal transparent windows on both sides

photons

sapphire LED sapphire

50 ~3 better than commercial thermo-electric coolers

1 0.9 GaAs LED at 280K (E = 1.43 eV) 0.8 g,LED GaAs PV cell at 330K 0.7 (Eg,PV = 1.41 eV) 0.6 Carnot COP = 5.6 0.5 0.4 0.3

(fraction of Carnot limit) of (fraction Carnot 0.2 Coefficient of performance of Coefficient 0.1 thermoelectric cooler ZT = 1 0 10-8 10-6 10-4 10-2 100 102 2 Cooling power density (W/cm ) 51 Thermos Conventional bottle Thermos refrigeration bottle GaAs (LED)

GaAs (PV)

Ag Ag

Vacuum Vacuum Ag Ag Conclusions: Since we are for the first time making really efficient photovoltaic cells (also LED's), some new ideas become very timely Beyond Solar

• High temperature Thermo-PhotoVoltaics, for cars and other vehicles

• Electro-luminescent refrigeration (thermophotonic cooling) LED PV cell Phonons Phonons and electro-luminescent heat engines.

+ + ϕLED V JLED JPV VPV LED Photon flux – –

• The automotive power market is 10 bigger than the solar panel electricity market.